Sources tell TMZ that Flav (real name: William Drayton Jr.) allegedly threatened his fiancee's teenage son with a knife in the midst of an explosive argument. (His fiancee, Liz, was reportedly involved in the dispute as well, and called 911.)

During the blowup, TMZ's sources explained that Flav was also physically abusive towards his fiancee. He was taken into police custody around 3 a.m.